The present invention relates to granular magnetite particles and a process for producing the same. Particularly, the present invention relates to granular magnetite particles assuming a black color and having a small magnetization, and a process for producing the same.
Granular magnetite particles according to the present invention are mainly intended as materials for coloring pigments for coatings, printing inks, rubber and plastics, and materials for magnetic toners, magnetic carriers and magnetic cards.
Since magnetite particles assume a black color, they are widely used as materials for coloring pigments for coatings printing inks, rubber and plastics.
In addition, since granular magnetite particles are ferromagnetic particles, they are mixed with and dispersed in a resin so as to produce composite particles which are used as material particles for magnetic toners and magnetic carriers for electrostatic copying.
Magnetite particles are sometimes dispersed in a vehicle so as to obtain a magnetic coating. The thus-obtained magnetic coating is directly applied to a card substrate or a magnetic tape obtained by applying the magnetic coating to a base film is adhered to a card substrate by an adhesive. In this way, magnetite particles are also used as material particles for magnetic cards such as credit cards provided with a magnetic stripe, railway tickets, railway season tickets, highway passes, telephone cards and railway cards.
In any of the above-described fields, there is no end to demand for higher capacity and higher quality of the products, and magnetite particles as a material are required to have improved properties, in particular, to have excellent dispersibility in a resin or a vehicle which facilitates the kneading of the particles with the resin or the mixing of the particles with the vehicle.
This fact is described with respect to coloring pigments for coatings, printing inks, rubber and plastics in Journal of the Japan Society of Color Materials, vol. 49, No. 1 (1976), page 8, "It seems to be no exaggerating to say that most properties wherein a coating has to possess, are determined by the dispersibility of the pigment in the coating film, in case where the pigment is the same. Theoretically, if the dispersibility of the pigment in the coating film is good, the color tone is vivid, and the fundamental properties intrinsic to the pigment such as a tinting strength and a hiding power are improved. In addition, a gloss, a definition, mechanical properties and an air permeability resistance of the coating film are also improved, which leads to the enhancement of the durability of the coating film. In this way, the dispersibility of a pigment in a coating film is an important factor which determines various properties of the coating film."
With respect to magnetite particles as a material for magnetic toners, Japanese Patent Application Laid-Open (KOKAI) No. 55-65406 (1980) states, ". . . (VII) The mixing degree of the magnetite particles with a resin should be high. The average particle diameter of a toner is ordinarily not more than several ten .mu.m, and the microscopic mixing degree in a toner is important for the properties of the toner . . . . "
With respect to magnetite particles as a material for magnetic carriers, Japanese Patent Application Laid-Open (KOKAI) No. 61-53660 (1986) states, "According to the present invention, . . . the dispersibility of primary particles is improved and the magnetic powder is uniformly dispersed, so that the variation of the charging property and the magnetic characteristics between the particles of a magnetic developing agent is reduced."
As to magnetite particles for magnetic cards, this fact is described in `Magnetic Cards and Chemical Technique` at pp. 68 to 72 of Journal of Chemical Technique, MOL (1985), published by Ohm-sha," . . . the properties are greatly changed with the state of dispersion of a magnetic material . . . . It is important to obtain a good dispersion system."
Magnetite particles are generally produced directly from an aqueous solution, namely, by a wet process, because this process makes it easy to produce discrete particles, in other words, particles having good dispersibility.
This wet process is a process of blowing an oxidizing gas in a temperature range of 60.degree. to 100.degree. C. into an aqueous solution which contains an Fe-containing precipitate such as Fe(OH).sub.2 and FeCO.sub.3, and which is obtained by mixing an aqueous ferrous salt solution such as ferrous sulfate and an aqueous alkaline solution such as sodium hydroxide and sodium carbonate (Japanese Patent Publication No. 44-668 (1969)).
However, although the granular magnetite particles obtained in the above-described wet process are discrete, the magnetization is as large as 55 to 70 emu/g, so that magnetic agglomeration is apt to be caused, which is an obstacle to an improvement of the dispersibility.
In addition, the conformity of the granular magnetite particles obtained by a wet process with a resin is very low due to the above-described magnetic agglomeration and the hydrophilic property of the particle surfaces, which increases the obstacle to an improvement of the dispersibility. This fact is described in Japanese Patent Application Laid-Open (KOKAI) No. 58-68754 (1983), "Especially, since fine magnetic particles used for one-component magnetic toners are generally hydrophilic, the dispersibility of the fine magnetic particles in a binder resin is very low, . . . which leads to a low picture quality."
Therefore, in various fields, granular magnetite particles having a small magnetization and subjected to hydrophobic treatment are sometimes in demand in order to enhance the dispersibility of the particles in a vehicle or a resin. This is demanded, especially, in the field of a magnetic toner.
In the case of using granular magnetite particles as the material particles for a magnetic toner, magnetite particles having a small magnetization and hydrophobic particle surfaces are sometimes in demand in order to realize a high picture quality.
With the development of a smaller-sized electrostatic copying machine having a higher capacity such as a higher speed, an improvement of the properties of a magnetic toner as a developer, in other words, a magnetic toner which is capable of developing with a high density and a high resolution has been strongly demanded.
The properties of a magnetic toner have a close relationship with various properties of the magnetic particles contained in a resin, and in order to enable development with a high density, it is necessary to increase the content of the magnetic particles contained in the resin. However, when the content of the magnetic particles is increased, the magnetic particles after development remain on the developed latent image as an agglomerate due to the magnetic agglomeration of the magnetic particles, so that it is difficult to reproduce a fine latent image with fidelity and it is impossible to obtain a high resolution.
In order to obtain a magnetic toner which enables development with a high fidelity and a high resolution, it is necessary to reduce the magnetization of the magnetic particles as much as possible, so that an increase in the magnetic particles in a resin does not cause magnetic agglomeration.
This fact is described in Japanese Patent Application Laid-Open (KOKAI) No. 4-184354 (1992), " . . . . In this way, a toner has been required which enables development with fidelity to signals and a manuscript, in other words, with fidelity to a latent image and, in addition, with a high density. However, it is difficult to satisfy such a demand by using a toner containing a magnetic material. For example, if the content of the magnetic material is merely increased in order to increase the coloring power so as to achieve a high density, the image quality is deteriorated and it is impossible to obtain a high resolution. This is because . . . the size of the spike is related to the magnetic force of the toner. If the content of a magnetic material is increased, the spike also becomes large. Such an spike is apt to generate an agglomerate on the latent image developed by the toner, so that it is difficult to reproduce a fine latent image with fidelity. On the other hand, in order to obtain a high resolution, the content of a magnetic material may be reduced so as to reduce the magnetic agglomeration force, however it leads to not only a reduction in the tinting strength but also a lowering in the developing power and the toner manufacturing efficiency due to an increase in the charge. However, combined use with a coloring agent such as carbon black as a countermeasure is difficult to put to practical use, because the environment dependency may be deteriorated.
. . To solve these problems, there are some proposals for adjusting the magnetic force so as to obtain a toner having a high performance . . . . " and " . . . It is possible to enhance the fine line reproducibility and obtain a high picture quality by development which has a high fidelity to a latent image and signals, by mixing an appropriate amount of magnetic material having an appropriately small magnetic force . . . . "
Since the surfaces of magnetic particles are hydrophilic, it is difficult to disperse the particles in a resin, and the contents of magnetic particles are nonuniform between magnetic toner particles. As a result, magnetic agglomeration is apt to be caused around the magnetic toner particles having a large content of magnetic particles.
In order to obtain a magnetic toner which enables development with a high density and a high resolution, it is strongly required that the magnetization of the magnetic particles is reduced as much as possible in order to prevent the generation of a magnetic agglomeration force even with a high content of the magnetic particles in a resin, and that the particle surfaces are hydrophobic in order to improve the dispersibility of the particles in the resin.
As magnetic particles having a small magnetization, a magnetic material composed of a metal oxide (concretely, zinc ferrite particles and zinc manganese ferrite particles) in which a magnetization .sigma..sub.s in a magnetic field of 1 kOe is 10 to 40 emu/g and the ferre-diameter in a horizontal direction is 0.05 to 0.5 .mu.m, is known (Japanese Patent Application Laid-Open (KOKAI) No. 4-184354 (1992)).
Various attempts at improving the properties of magnetite particles have also been conducted. For example, there are known a process for producing an Fe.sub.3 O.sub.4 iron oxide pigment having an excellent particle distribution and an improved oxidation stability, which comprises the steps of adding not less than one equivalent of an aqueous alkaline solution to a ferrous salt solution, and carrying out an air oxidization, thereby producing a spinel type Fe.sub.3 O.sub.4 iron oxide while adding a hydroxosilicate solution or a hydroxoaluminic acid solution to the solution in the oxidization process of the produced ferrous hydroxide to the Fe.sub.3 O.sub.4 iron oxide (Japanese Patent Publication No. 1-36864 (1989)); and a toner having excellent stability and blocking resistance, which comprises a magnetic powder containing 0.05 to 10 wt % of aluminum element, and a binder resin in which the content of THF insolubles are less than 10 wt %, and the molecular weight distribution of the THF solubles in GPC has a specified range (Japanese Patent Application Laid-Open (KOKAI) No. 4-190242 (1992)).
Although granular magnetite particles assuming a black color and having a small magnetization, or granular magnetite particles assuming a black color, and having a small magnetization and hydrophobic particle surfaces are now in the strongest demand, such granular magnetite particles have not been produced yet.
The zinc ferrite particles or the zinc manganese ferrite particles described in Japanese Patent Application Laid-Open (KOKAI) No. 4-184354 (1992) have a small magnetization, but since the hue thereof is not black but brown, so that it is impossible to obtain a magnetic toner which enables development with a high density. Further, there is no description nor suggestion of the production of particles assuming a black color in Japanese KOKAI 4-184354.
The granular magnetite particles obtained by the process described in Japanese Patent Publication No. 1-36864 (1989) or Japanese Patent Application Laid-Open (KOKAI) No. 4-190242 (1992) assume a black color which is characteristic of magnetite particles, but the magnetization thereof is so large as to be apt to cause magnetic agglomeration, as shown in Comparative Example 1 which will be described later.
Accordingly, the present invention aims at solving the above-described problems in the related art and providing granular magnetite particles assuming a black color and having a small magnetization.
As a result of studies undertaken by the present inventors so as to solve these problems, it has been found that the granular magnetite particles obtained by blowing an oxygen-containing gas into an alkaline suspension which contains a precipitate containing aluminum and iron, which has a pH of not less than 10, and which is obtained by adding 15 to 70 mol % of an aluminum compound (calculated as Al) based on the ferrous iron in an aqueous ferrous salt solution to an aqueous alkaline solution and then adding the aqueous ferrous salt solution to the thus-obtained mixture, assume a black color and have a magnetization of not more than 50 emu/g in a magnetic field of 1 kOe. The present invention has been achieved on the basis of this finding.